Copy and paste is all well and good, but it's a four-step procedure. When you want to copy a region into a nearby spot, there's a much more efficient technique: Option-dragging.
Press the Option key (called Alt on non-U.S. keyboards). While it's down, drag a region to a new position. You peel off a perfect copy of the original. (You may recognize this technique from working with icons in the Finder.)